Recirculation of a portion of internal combustion engine exhaust gas to the engine fresh air intake, generally termed exhaust gas recirculation EGR, reduces engine production and emission of oxides of nitrogen NOx by decreasing the level of oxygen in the engine combustion process, and by reducing the capacity of the engine intake air charge to absorb heat, thereby lowering combustion temperature and frustrating NOx production. The amount of EGR must be closely controlled as too much EGR can significantly reduce engine performance and can actually increase the level of undesirable engine emissions. Accordingly, sophisticated EGR control systems have been developed, for example including precision EGR valves for varying a degree of opening of an exhaust gas conduit positioned between the engine exhaust gas path and the engine fresh air intake path.
The precision EGR valve necessarily must operate in a harsh environment characterized by temperature extremes, vibration, and various contaminants. Despite such harsh operating conditions, the EGR valve is required to maintain a high degree of control precision so that engine emissions may be minimized under many varying engine operating conditions. Likewise, other EGR system components such as the EGR conduit through which the exhaust gas flow and an EGR valve position sensor must remain "healthy" to maintain the integrity of the EGR system. In the event an EGR system component fails to operate as expected, corrective action must be taken as soon as possible, as engine performance and emissions may be negatively affected until the failure is remedied. Any significant EGR system failure that may impact the effectiveness of the system must be diagnosed in a reasonable amount of time and reported so that a remedy may be rapidly applied.
Misdiagnosis of an EGR system fault. condition can result in inconvenient and wasteful fault treatment procedures including unnecessary replacement of EGR system components. Any EGR system or component diagnostic approach must therefore be highly accurate, wherein any failure reported by the approach is associated with a very low potential for misdiagnosis.
EGR diagnostic approaches have been proposed which consume significant engine controller processing time and which add significant engine controller throughput burden. Further, proposed diagnostic approaches are prone to misdiagnosis. Still further, proposed diagnostic approaches only return reliable diagnosis under certain specific operating conditions. If the operating conditions are not present, no diagnostic is available. Still further, proposed intrusive diagnostic approaches may appreciably reduce engine performance or significantly increase engine emissions, or may cause sudden perceptible disturbances that may reflect poorly on engine or vehicle stability.
Accordingly, it would be desirable to provide an EGR system diagnostic that requires minimum processor time and adds minimal additional processor throughput burden yet accurately diagnoses the EGR system under a variety of commonly occurring engine operating conditions with minimum disruption to vehicle operations.